Profound bradycardia is a common complication in the early posttraumatic period following cervical spinal cord damage. It is thought to be due to temporary inactivity of the sympathetic nervous system after separation from supraspinal control, coupled with unopposed parasympathetic dominance because of vagus nerve sparing. Hypoxia, underventilation, and tracheal suctioning appear to intensify the bradycardia. This study examined the effect of vagal stimulation using facial immersion and apnea on heart rate in patients with quadriplegia. Ten patients with quadriplegia (eight males and two females, mean age 25, age range 16-37) and ten healthy controls (eight males and two females, mean age 25, age range 15-37) underwent facial immersion during ECG monitoring. The patients with quadriplegia were studied at a mean of 9.4 months after injury (range, 4-26 months); cord level of injury ranged from C5 to C8; seven of the ten had had heart rates of less than 50 beats/min in the early period after injury and five of the ten had received treatment for this. No change in heart rate was seen in the quadriplegic group during facial immersion (delta = 1.5 +/- 2.7 beats/min, mean +/- SE), but a significant drop in heart rate was seen in the control group (delta = -12.2 +/- 3.7 beats/min, mean +/- SE; p less than 0.01). No significant decreases were seen in either group during apnea alone. Patients with chronic quadriplegia appear to have less sensitivity than controls to the vagal-stimulating maneuver of facial immersion and apnea.